T wo degrees Celsius: the point of no return. Once average global temperatures exceed preindustrial levels by this amount, scientists warn, a climate catastrophe could become inevitable. Current projections indicate that it would be too late to prevent sea ice from disappearing, ice sheets from collapsing and rising seas from swallowing heavily populated coastlines.

A whole new climate would emerge. Lasting millennia, this hotter world would be humankind’s most enduring legacy, perhaps outlasting our species. Fears over this new climate order spurred more than 100 nations to sign the Copenhagen Accord, drafted in 2009. Althou